A Mach-Zehnder type (MZ type) optical modulator has the structure that light incident upon an optical waveguide is branched into two waveguides by an optical branching filter and the branched lights are propagated in a constant length, which thereafter, are again multiplexed by an optical multiplexer. The two branched optical waveguides are respectively provided with phase modulators each of which changes a phase of light propagating in each of the optical waveguides to change an interference condition of the lights to be multiplexed, thus modulating intensity of the light or a phase of the light.
An example of a material configuring the optical waveguide in the Mach-Zehnder type (MZ type) optical modulator may include a dielectric body of LiNbO3 or the like, or a semiconductor of InP, GaAs, Si or the like. When a voltage is applied to the optical waveguide by traveling wave electrodes arranged in the vicinity of the optical waveguide, the phase of the light is changed. In regard to a principle of changing the phase of the light, primarily a Pockels effect is used in LiNbO3, a Pockels effect and a quantum confined stark effect (QCSE) are used in InP and GaAs, and a carrier plasma effect is used in Si.
For performing optical communications at high speeds and in low-consumption power, it requires an optical modulator high at modulating speeds and low in a driving voltage. For performing optical modulation at high speeds of 10 Gbps or more and in an amplitude voltage of several volts, it requires traveling wave electrodes of matching and propagating a high-speed electrical signal and a speed of the light propagating in the phase modulator and performing a mutual function thereof. At present, there has been put into practice an optical modulator with the traveling wave electrode an electrode length of which has several millimeters to several ten millimeters (for example, NPL 1). The optical modulator using the traveling wave electrode is required to have an electrode structure and an optical waveguide structure that are low in an optical loss and small in reflection in such a manner as to be capable of propagating the electrical signal and the light propagating in the waveguide without reducing the intensity thereof.
An example of the MZ type modulator may include a silicon optical modulator an optical waveguide of which is formed of silicon. The silicon optical modulator is produced such that a fine line of Si is processed in such a manner that light can wave-guide on a silicon-on-insulator (SOI) layer from an SIO substrate in which a thin film of Si is attached on an oxide film (BOX) layer a surface of an Si substrate of which is thermally-oxidized, dopants are thereafter injected into the processed fine line to become a p-type/n-type semiconductor, and deposit of SiO2 to be clad layers of light, formation of traveling wave electrodes and the like are performed. At this time, the waveguide of the light is necessary to be designed and processed to make the optical loss small. The doping for p-type and n-type of the processed fine line and production of the traveling wave electrode are required to be designed and processed to control loss generation of the light to be small and reflection and a loss of a high-speed electrical signal to be small.
FIG. 1 is an upper surface transparent diagram illustrating the configuration of a conventional MZ type optical modulator 100. The MZ type optical modulator 100 is a silicon optical modulator, and includes an input optical modulator 101, an optical branching filter 102 for branching the light incident from the input optical modulator 101 in a ratio of 1:1, and optical waveguides 103 and 104 upon which light is incident from the optical branching filter 102. The MZ type optical modulator 100 includes a phase modulation portion 111 that modulates a phase of light propagating in the optical waveguide 103, a phase modulation portion 112 that modulates a phase of light propagating in the optical waveguide 104, an optical waveguide 105 for propagating the light from the phase modulation portion 111, and an optical waveguide 106 for propagating the light from the phase modulation portion 112. The MZ type optical modulator 100 includes an optical multiplexer 107 that multiplexes the lights, each phase of which is modulated, from the optical waveguides 105 and 106, and an output optical waveguide 108 that outputs the light multiplexed by the optical multiplexer 107.
The phase modulation portion 111 includes traveling wave electrodes 121 and 122 extending in an x axis direction, and an optical waveguide 123, and by applying a voltage to the traveling wave electrodes 121 and 122, changes a phase of light that wave-guides in the optical waveguide 123. The phase modulation portion 112 includes traveling wave electrodes 124 and 125 extending in the x axis direction, and an optical waveguide 126, and by applying a voltage to the traveling wave electrodes 124 and 125, changes a phase of light that wave-guides in the optical waveguide 126. The optical waveguides 123 and 126 each have the structure called a rib waveguide different in thickness, are formed of Si and are provided with SiO2 clad layers formed upward and downward.
FIG. 2 is a cross section illustrating the phase modulation portion 111 of the conventional MZ type optical modulator 100 described in FIG. 1, taken in arrows II-II. FIG. 2 is a cross section illustrating the phase modulation portion 111 in a direction (y-z plane) vertical to a waveguide direction (x axis direction) of light, and the phase modulation portion 111 includes an Si substrate 201, and the optical waveguide 123 formed on the Si substrate. The optical waveguide 123 includes a first SiO2 clad layer 202 on the Si substrate 201, an Si semiconductor layer 203 on the first SiO2 clad layer 202, and a second SiO2 clad layer 204 on the Si semiconductor layer 203. Third clad layers 205, 206 are formed on both sides of the Si semiconductor layer 203. It should be noted that the phase modulation portion 112 also has the same configuration.
The optical waveguide 123 has the structure of the rib waveguide, and the Si semiconductor layer 203 in which light wave-guides is interposed between the first SiO2 clad layer 202 and the second SiO2 clad layer 204. The Si semiconductor layer 203 includes a rib portion A0 that is arranged in an Si semiconductor layer region thick in the center as a core of the optical waveguide 123, and a first slab portion A1 and a second slab portion A2 that are arranged in both sides of the rib portion A0 and are Si semiconductor layer regions thinner than the rib portion A0. The optical waveguide 123 confines the light using a difference in a refraction index between the Si semiconductor layer 203, and the first SiO2 clad layer 202 and the second SiO2 clad layer 204 in the periphery of the Si semiconductor layer 203.
The traveling wave electrode 121 is formed in the x axis direction on an upper surface in an end of the Si semiconductor layer 203 at the opposite side to the rib portion A0 of the first slab portion A1, and the traveling wave electrode 122 is formed in the x axis direction on an upper surface in an end of the Si semiconductor layer 203 at the opposite side to the rib portion A0 of the second slab portion A2.
The Si semiconductor layer 203 has conductivity by doping of atoms such as boron (B), phosphorous (P) or arsenic (As) to Si with a method of implantation of ions or the like. Here, the Si semiconductor layer 203 is formed of four regions that are different in a doping concentration. An end of the first slab portion A1 in the Si semiconductor layer 203 at the opposite side to the rib portion A0 becomes a high-concentration p-type semiconductor region 203-3, and an end of the second slab portion A2 in the Si semiconductor layer 203 at the opposite side to the rib portion A0 becomes a high-concentration n-type semiconductor region 203-4. The Si semiconductor layers 203 at the rib portion A0-side of the first slab portion A1 and at the first slab portion A1-side of the rib portion A0 become an intermediate-concentration p-type semiconductor region 203-1. The Si semiconductor layers 203 at the rib portion A0-side of the second slab portion A2 and at the second slab portion A2-side of the rib portion A0 become an intermediate-concentration n-type semiconductor region 203-2.
A boundary of the high-concentration p-type semiconductor region 203-3 makes contact with a boundary of the intermediate-concentration p-type semiconductor region 203-1, and a boundary of the high-concentration n-type semiconductor region 203-4 makes contact with a boundary of the intermediate-concentration n-type semiconductor region 203-2. The boundaries may overlap to be subjected to doping. The rib portion A0 has a p-n junction structure in which the intermediate-concentration p-type semiconductor region 203-1 makes contact with the intermediate-concentration n-type semiconductor region 203-2. The other example may include a p-i-n junction structure in which an i-type (intrinsic) semiconductor region is interposed between the intermediate-concentration p-type semiconductor region 203-1 and the intermediate-concentration n-type semiconductor region 203-2.
The traveling wave electrode 121 is connected to the high-concentration p-type semiconductor region 203-3, and traveling wave electrode 122 is connected to the high-concentration n-type semiconductor region 203-4. An inversely-biased electrical field is applied to the p-n junction portion or the p-i-n junction portion by the traveling wave electrodes 121 and 122 to change a carrier density in the inside of the rib portion A0 of the Si semiconductor layer 203 and change a refraction index of the Si semiconductor layer 203 (carrier plasma effect), making it possible to modulate the phase of the light.
Since a dimension of the Si semiconductor layer 203 depends on refraction indexes of materials as the core and clad, it cannot be determined uniquely. Referring to one example thereof, generally in a case of having the rib waveguide structure provided with the rib portion A0, and the slab portions A1 and A2 at both the sides of the rib portion A0 in the Si semiconductor layer, the dimension has approximately a core width 400 to 600 nm (a rib width of the Si semiconductor layer 203)×a height 150 to 300 nm×a slab thickness 50 to 200 nm×a length of several mm in the Si optical waveguide 123.
The optical modulator is required to be small in an optical loss for transmitting the modulated optical signal for a long distance. Since a part of the light propagated by carriers such as electrons/holes is absorbed in the p-type/n-type doped conductive semiconductor layer in the optical waveguide, it is necessary to set a condition of the doping in such a manner as to control the carrier concentration to be a constant value or less to suppress the optical loss. In regard to a carrier density in the doped region, the carrier density is approximately 1020 [cm−3] in a high-concentration p-type semiconductor region 203-3 (p++), the carrier density is approximately 1017-18 [cm−3] in an intermediate-concentration p-type semiconductor region 203-1 (p+), the carrier density is approximately 1017-18 [cm−3] in an intermediate-concentration n-type semiconductor region 203-2 (n+) and the carrier density is approximately 1020 [cm−3] in a high-concentration n-type semiconductor region 203-4 (n++).
The light is confined in the Si semiconductor layer 203 higher in a refraction index than the SiO2 clad layers 202 and 204 in the periphery of the Si semiconductor layer 203, and propagates in the x axis forward direction in FIG. 1. The high-concentration p-type semiconductor region 203-3 and the high-concentration n-type semiconductor region 203-4 each are provided to secure conduction having a small contact resistance with the traveling wave electrode and to suppress electrical resistances of the semiconductor layers themselves configuring the Si semiconductor layer 203 to be small. On the other hand, a carrier density of the intermediate-concentration p-type semiconductor region 203-1 and a carrier density of the intermediate-concentration n-type semiconductor region 203-2 configuring the rib portion A0 as the core are set to be lower than that of the high-concentration p-type semiconductor region 203-3 and that of the high-concentration n-type semiconductor region 203-4. This is because since the carrier generated by the doping absorbs the light, the doping concentration is required to be lowered to reduce the optical loss. By lowering the doping concentration, the optical loss in the optical waveguide is 3 dB/cm in a passive optical waveguide not doped, and on the other hand, can be suppressed to be approximately 6 dB/cm. Since a field of the light propagating in the rib portion A0 of the Si semiconductor layer 203 is distributed to leak also outside of the region of the rib portion A0, when the high-concentration p-type semiconductor region 203-3 and the high-concentration n-type semiconductor region 203-4 are arranged to be close to the rib portion A0, the optical loss of the optical waveguide 123 increases. Accordingly, for preventing the high-concentration p-type semiconductor region 203-3 and the high-concentration n-type semiconductor region 203-4 from being arranged to be close to the rib portion A0, it is preferable that a distance wpn between the high-concentration p-type semiconductor region 203-3 and the high-concentration n-type semiconductor region 203-4 is 1600 nm or more.
For realizing an optical modulator that is fast in a modulation speed, low in an optical loss and low in a drive voltage, it is required to realize an optical waveguide structure that is low in a loss and low in reflection to be capable of propagating the light without reducing intensity of the light in the waveguide, to cause an operation frequency band to be a high frequency, and to lower an phase inversion voltage Vπ.
Here, in the MZ type optical modulator, a modulation efficiency is found by a phase inversion voltage Vπ×a length L of a traveling wave (phase modulation) electrode. Then, when the length L of the traveling wave electrode is made short without changing the modulation efficiency to make the optical loss small, the phase inversion voltage Vπ is made large and the drive voltage increases. On the other hand, there is a tradeoff relation that when the phase inversion voltage Vπ is small, the length L of the traveling wave electrode is large to increase the optical loss of the phase modulation portion. Therefore, for realizing the optical modulator low in an optical loss and low in a drive voltage, it is necessary to enable a high-speed operation even if the length L of the traveling wave electrode is long. When the length L of the traveling wave electrode can be set long, it is not necessary to increase the phase inversion voltage Vπ to be large.
For suppressing the optical loss of the optical waveguide 123 in the conventional MZ type optical modulator 100 in FIG. 1, the light is required to be confined in the rib portion A0 of the Si semiconductor layer 203 as the core of the optical waveguide 123. For confining the light in the rib portion A0, it is required to have the rib structure in which the first slab portion A1 and the second slab portion A2 in both the sides of the rib portion A0, which are sections where the light leaks out of the rib portion A0, are thinned. As a result, there occurs a problem that a resistance value of the Si semiconductor layer 203 cannot be lowered by thickening the intermediate-concentration p-type semiconductor region 203-1 and the intermediate-concentration n-type semiconductor region 203-2, and the phase inversion voltage Vπ cannot be made small.
In addition, as described above, since the doping concentration in the vicinity of the rib portion A0 (core of the optical waveguide) in the Si semiconductor layer 203 cannot be made to a high concentration, an electrical resistance value of the p-n junction portion or the p-i-n junction portion of the semiconductor configuring the Si semiconductor layer 203 cannot be lowered largely. Therefore the resistance value of the Si semiconductor layer 203 causes a loss of the high-frequency electrical signal and the voltage to be applied to the p-i-n junction portion or the p-n junction portion is attenuated, posing a problem that the phase inversion voltage Vπ cannot be made small.
The present invention is made in view of this problem, and an object of the present invention is to provide an MZ type optical modulator that can simultaneously realize requirements of being fast in a modulation speed, low in an optical loss and low in a drive voltage.